This invention relates to a method and apparatus for rapid formation of a highly uniform nonwoven web of staple fibers and is particularly suitable for the formation of the low basis weight webs of thermoplastic fibers at a high rate of speed.
Nonwoven fabrics are produced by a variety of methods, and in general, such methods involve the continuous laydown of fibers or filaments in the form of an unconsolidated flat web on a conveyor, followed by consolidation of the web, such as by bonding or locking the fibers together to form the web into a cohesive fabric.
The carding of staple fibers into an unconsolidated web followed by point bonding with a hot calender is one well known method of producing a nonwoven fabric. In such a process the fibers, which are received in bales, are first opened with standard textile opening equipment. The opened fibers are then fed to single or multiple cards which are installed in line, each forming a thin web. The webs are then layered together, then usually spread to increase web width, and fed to a hot calender for thermal bonding. The customary calender consists of two heated rolls, one being a smooth steel anvil roll, the other being a roll with an embossed pattern. The high points of the pattern are the area where the fibers are bonded together through partial melting. Such systems can produce webs which are reasonably uniform at a given speed and basis weight. Typically, a reduction in unit weight or an increase in speed results in a noticeable degradation in the uniformity of the fiber distribution. More precisely, at lower basis weights the web develops a more blotchy appearance due to areas of higher and lower concentrations of fibers. In the worst case, holes will form where the concentration of fiber is low. The degradation in web uniformity for the traditional system is also linked to the need of additional draw on the unbonded web to eliminate the bulging of the web which would otherwise occur at various points in the process. The amount of draw used to control the web during transport to the calender is inversely proportional to the cohesion of the unbonded web. A low cohesion web will require a higher draw. The spreading section and the calender nip point are prime areas where the bulging occurs. This bulging, if not eliminated, causes extremely poor web uniformity. A lighter web, when submitted to such increase in draw, develops even greater defects because the extremely light areas are now deformed into holes in the web.
The prior art has tried to minimize the requirement for draw by using equipment transfer geometry and higher cohesion fiber to produce nonwoven material at higher production speeds. Both modifications have produced only moderate improvements in speed or uniformity.
Other prior art has been the development of a machine which reorganizes the carded unbonded web (with minimal or no increase in output speed) by reforming it on a vacuum collector such as described in U.S. Pat. No. 4,475,271. This process can produce a web with a more uniform balance in tensile strength between the MD and CD direction but, it does not deliver the desired level of uniformity in fiber distribution as judged by visual appearance.